Medical imaging using x-rays is sensitive to differences in the density and composition of tissues such as bone, muscle, and fat. Many diseases, however, are only poorly detected or are completely undetectable in this manner. Molecular imaging has the potential to increase disease detection sensitivity and specificity through the use of molecular probes, or biomarkers, designed to bind to specific biological targets, such as proteins in the body that signal cellular events, such as apoptosis, angiogenesis, hypoxia, and other disease markers. A few contrast agents, such as iodine or barium sulfate, can be imaged with x-rays; however, the low sensitivity of these contrast agents requires significant (often toxic) doses of contrast agents to identify injected contrasts. Thus, x-ray imaging alone is intrinsically unable to perform molecular imaging. Other types of molecular imaging use radiotracers as biomarkers, i.e., radioactive materials that bind to a specific biological targets, and special detectors to sense the ionizing radiation emitted from these radioactive biomarkers. These also involve significant radiation dose.
Current optical molecular imaging use optical excitation with optical emission. For example, there are several instruments that image molecular markers using optical fluorescence. These systems use optical light, e.g., from a laser, to excite fluorescent dyes attached to molecular targets. Optical molecular imaging has also been proposed using up-conversion phosphors. In these techniques, the phosphors are irradiated with infrared energy from a laser, causing them to produce optical light at a lower wavelength. Unfortunately, due to the absorption of the exciting infrared light by water/tissue, combined with the absorption of the emitted optical light from the phosphors, these optical molecular imaging techniques do not provide a strong signal for imaging.